Fix typos and grammar in documentation and code comments

Comment fixes are applied on HEAD, and documentation improvements are
applied on back-branches where needed.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20210408164008.GJ6592@telsasoft.com
Backpatch-through: 9.6

src/backend/commands/analyze.c

@@ -617,7 +617,7 @@ do_analyze_rel(Relation onerel, VacuumParams *params,
 	 *
 	 * We assume that VACUUM hasn't set pg_class.reltuples already, even
 	 * during a VACUUM ANALYZE.  Although VACUUM often updates pg_class,
-	 * exceptions exists.  A "VACUUM (ANALYZE, INDEX_CLEANUP OFF)" command
+	 * exceptions exist.  A "VACUUM (ANALYZE, INDEX_CLEANUP OFF)" command
 	 * will never update pg_class entries for index relations.  It's also
 	 * possible that an individual index's pg_class entry won't be updated
 	 * during VACUUM if the index AM returns NULL from its amvacuumcleanup()

src/backend/commands/cluster.c

@@ -1422,7 +1422,7 @@ finish_heap_swap(Oid OIDOldHeap, Oid OIDNewHeap,
 								 PROGRESS_CLUSTER_PHASE_FINAL_CLEANUP);
 
 	/*
-	 * If the relation being rebuild is pg_class, swap_relation_files()
+	 * If the relation being rebuilt is pg_class, swap_relation_files()
 	 * couldn't update pg_class's own pg_class entry (check comments in
 	 * swap_relation_files()), thus relfrozenxid was not updated. That's
 	 * annoying because a potential reason for doing a VACUUM FULL is a

src/backend/commands/copyfrom.c

@@ -410,7 +410,7 @@ CopyMultiInsertBufferCleanup(CopyMultiInsertInfo *miinfo,
  * Once flushed we also trim the tracked buffers list down to size by removing
  * the buffers created earliest first.
  *
- * Callers should pass 'curr_rri' is the ResultRelInfo that's currently being
+ * Callers should pass 'curr_rri' as the ResultRelInfo that's currently being
  * used.  When cleaning up old buffers we'll never remove the one for
  * 'curr_rri'.
  */